The present invention relates to a blade material useful in an electrophotographic printing apparatus, including image on image, contact electrostatic printing, and digital apparatuses. Specifically, the present invention relates to a blade material useful in a cleaning blade, in particular a blade for cleaning an imaging member, used therein to remove particles, especially non-agglomerated particles, adhering to the charge-retentive, image bearing or photoconductive surface.
In the process of electrophotographic printing, an imaging surface is charged to a substantially uniform potential. The imaging surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the imaging surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the imaging surface to a sheet of support material and permanently affixed thereto. In a manner similar to the aforementioned dry toner imaging, liquid toner-based electrophotographic printing produces visible images from latent electrostatic images that are then transferred and fixed. In a unique liquid toner-based printing technology referred to as Contact Electrostatic Printing, the image is formed by selective "transfer" while in direct contact between imaging surface and image bearing surface. The imaging surface can be a photoreceptor or dielectric, and the image-bearing member is a compliant member, similar to an offset press blanket.
In a reproduction process of the types as described above, it is inevitable that some residual toner will remain on the imaging surface or image bearing surface after the toner image has been transferred to the sheet of support material (e.g., paper). It has been found that with such a process, the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces, and therefore, some of the particles remain on the surface after transfer of the toner image. In the process of Contact Electrostatic Printing (CEP), the development step produces a residual layer of liquid developer that is the "negative" of the image area. Typically, the CEP development process produces a residual image with greater mass than the imaged area. The residual must be removed from the imaging surface and reclaimed after each revolution. The residual material removed per unit time is much greater in the case of CEP than conventional dry toner development processes. In addition to the residual toner, other particles, such as paper debris (i.e. Kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer, or development in the case of CEP. These residual particles are different from agglomerated particles in that they are not groups of particles that have built up over time. Hereinafter, the term "residual particles" encompasses residual toner and other residual particles remaining after image transfer. The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid interfering with recording a new latent image thereon.
Various methods and apparatuses may be used for removing residual particles from the imaging surface. Hereinbefore, a cleaning brush, a cleaning web, and a cleaning blade have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface.
In addition to forming residual particles, dry toner particles agglomerate with themselves and with certain types of debris such as paper fibers, dirt and the like, thereby forming spot-wise depositions that eventually strongly adhere to the image bearing member. These spot depositions range from 50 micrometers to greater than 400 micrometers in diameter, but typically are about 200 to about 250 micrometers in diameter, and 5 to 25 micrometers in thickness, but typically about 5 to 15 micrometers in thickness. The agglomerates range in material compositions from toner by itself to a broad assortment of plastics and debris from paper. The spots may appear at random positions on the surface of the image-bearing member. Because the spot material is charged when passing under the charge corotron, the toner is subsequently developed on it. When the image is developed and subsequently transferred to a copy substrate, the toner on the spot is also transferred to the copy substrate. Accordingly, the spots cause a copy quality defect showing up as a black spot on a background area of the copy, which is the same size as the spot on the image-bearing member. The spot on the copy varies slightly with the exact machine and the specific operating conditions, but cannot be deleted by controlling the machine process controls.
For removing residual particles for both liquid and dry image forming processes, a relatively "soft" cleaning blade has been used in the past. Such a "soft" blade was necessary in order for the blade to uniformly tuck for efficient cleaning. The force required to cause the blade to tuck uniformly is the minimum cleaning force. Soft cleaning blades are made from a soft polyester urethane material having a hardness of from about 60 to about 80 Shore A, and on average have a hardness of about 70 Shore A. Also, the soft materials have a very "high" coefficient of friction. The high coefficient of friction usually ranges from about 25 to about 200 when measured at about 30.+-.5 RH (percent relative humidity) and 72.+-.2.degree. F. The high friction can cause the blade to tuck severely when the blade contacts a clean portion of the imaging member. This, in turn, causes a random failure mode. This severe tucking stresses the cleaning edge and creates stress fractures. The stress fractures eventually develop into craters. These craters increase in size as use of the blade continues, and an increase in the occurrence of nicks in the cleaning edge occurs. Field studies determined that stress fractures, craters, and nicks accounted for about 80 percent of the blade failures for one Xerox machine.
Efforts at improving the cleaning efficiency of a soft cleaning blade in the dry toner process, include providing lubrication to aid in decreasing the friction of the blade. Also, with "soft" cleaning blades, blade squeal occurs when the lubrication level is low, especially at high temperatures of about 80.degree. F. Blade squeal creates a high pitch noise from the machine that annoys users and people working in the office environment. This is primarily a concern on copier/printers, which use drums as image bearing members. There are several methods that can be employed to reduce the blade squeal. Damping features can be attached to the image-bearing member drum cavity, and the blade thickness and extension can be adjusted to reduce the noise. The noise is caused by the high frequency vibration of the cleaning edge on the imaging surface and occurs when the friction is too low. Another problem associated with "soft" blades is that the blade tends to stick-slip on the imaging surface in the absence of lubrication, thereby severely stressing the cleaning edge and causing the blade to miss residual particles to be cleaned. In a liquid system, the blade is immersed in liquid carrier that provides the lubrication. The stick-slip phenomena apply mainly to dry toner imaging.
Turning to a spots blade useful in removing agglomerated particles formed in the dry process, several copier products commonly use a "hard" urethane blade material (supplied by Acushnet and Zatec) as a spots blade. The spots blade is positioned, after or downstream from the cleaning station, to remove agglomerates and debris from the image-bearing member. The purpose of the spots blade is not for removing toner, but for removing agglomerated spots. Therefore, the set up parameters for the spots blade (for example, the blade load and angles) are different from a standard cleaning blade. As set forth above, with the standard "soft" cleaning blade, the blade force and angles are set so that the cleaning edge slides on the image-bearing member to clean toner, and this set-up results in the cleaning edge sliding in a tuck configuration. Alternatively, for the spots blade, the load and angles are set so that the blade does not tuck, but slides on the image-bearing member and "bumps" or "knocks" the spots off the image bearing member. Therefore, spots blades are made of "hard" materials such as polyurethanes having a hardness of from about 80 to about 95 Shore A. Preferred spots blades are positioned at a low angle of attack in engagement with the charge retentive surface.
U.S. Pat. No. 5,339,149 discloses a spots blade made of a polyester urethane having a low coefficient of friction, low resilience, and a hardness of from about 80 Shore A to about 90 Shore A.
U.S. Pat. No. 5,416,572 discloses a spots blade made of a polyurethane material having a hardness of 80 Shore A.
U.S. Pat. No. 5,349,428 discloses a spots blade positioned at a low angle of attack relative to the photoreceptor to minimize tuck occurrence. The spots blade is made of a polyurethane material having a hardness of 80 Shore A.
U.S. Pat. No. 4,989,047 discloses a polyurethane spots blade material having a hardness of 70 Shore A. A relatively low load is applied to the blade and it is positioned at a low angle of attack relative to the photoreceptor.
U.S. Pat. No. 5,031,000 discloses a polyurethane spots blade material having a hardness of 70 Shore A. The blade is supported in a floating support assembly to prevent tuck-under and damage to the blade.
U.S. Pat. No. 5,732,320 discloses a relatively hard spots blade made of polyether urethane, and in preferred embodiments, having a hardness of from about 86 to about 100 Shore A.
Therefore, relatively "soft" blades have been used to clean residual toner particles, and relatively "hard" blades have been used as spots blades for cleaning agglomerated toner from an imaging surface.
It is desirable to provide a cleaning blade for cleaning the imaging member, which has the superior properties of both "hard" and "soft" blades, and which dispenses with the need for both a cleaner and a spots blade. It is desirable to provide a cleaning blade with increased cleaning efficiency without the need for lubricants. It is also desirable to provide a cleaning blade that does not exhibit stick-slip motion on the imaging surface, thereby stressing the cleaning edge, and missing residual particles to be cleaned. Moreover, it is desirable to provide a cleaning blade that is tough and has increased strength, and is therefore less resistant to tearing. These factors give the blade very high reliability. The desirable overall qualities are excellent cleaning, and high reliability for cleaning dry toners and liquid inks.